Recrystallization of an Unknown Solid

Background

Crystalline compounds are generally purified via recrystallization.
In a recrystallization, we dissolve the impure solid in a minimum
amount of hot solvent and then allow crystals to form slowly. It is
important to choose a solvent that will not dissolve the substance at
room temperature so that the pure crystals may be recovered. If too
much solvent is used, the recovery of sample will be decreased. If
the solvent is not hot when the dissolution is carried out, too much
solvent will be used, leading to diminished yield.

This purification method takes advantage of the differences in
solubility between the compound and its impurities. Most of the
impurities will remain dissolved in the cool solvent, allowing them to
be removed when the sample is isolated by vacuum filtration. Some of
the impurities may not dissolve even in hot solvent, requiring a hot
filtration to remove them.

Occasionally the crystals may not form as the solution cools. A
few steps can be taken to induce crystallization in these cases.
These include: (a) using a seed crystal, (b) scratching the inside
of the flask, (c) cooling the mixture in an ice bath, and (d) removing
some of the excess solvent by boiling, then allowing the mixture to
cool again. Seed crystals and flask scratching both induce
crystallization by providing a surface on which the crystals can begin
forming. It is important to always save some impure sample to
use as seed crystals.

Once the crystals have formed, the sample should be cooled in an
ice bath to maximize the recovery. The crystals are then isolated
using vacuum filtration. In organic lab, these vacuum filtrations are
typically carried out using a Hirsch funnel and a filter flask, as
shown below. You should always use a clean filter flask (instead of
using one left behind by a classmate), in case you need to re-filter
the sample or take a second crop of crystals. Finally, you should
wash your crystals with a small amount of cold solvent
to wash away any impurities. Before you obtain a melting point or
weight of your sample, be sure that the sample has dried adequately.
Remaining solvent will give erroneously high weights, and it is an
impurity that will influence the physical properties of your product.

Melting points

You should remember the discussion of colligative properties from
CHE 132. (If not, reread that chapter in your general chemistry
textbook.) Colligative properties, like boiling point elevation and
melting point depression, depend on the number of solute particles in
a solvent. Using salt to melt ice on the roads is an example of a
colligative property. Just as salt lowers the melting point of the
ice, impurities lower the melting points of organic compounds. Thus,
the purity of a compound may be qualitatively assessed by
taking its melting point. In addition to lowering the melting point,
impurities also widen the melting point range. You should
always report the entire melting range of a sample for
that reason. Likewise, it is good practice to compare your
experimentally determined melting point with the literature melting
point of the pure compound.

Procedure

Heat a beaker of water to a gentle boil on a hot plate to use as a
heating medium. You will first need to determine the solubility of
your unknown in several common solvents (water, ethanol, petroleum
ether, dichloromethane, and ethyl acetate). Place about 20 mg (a
small spatula-tip full) of finely crushed unknown in each of several
test tubes and add about 0.5 mL of each solvent to the different tubes
containing the solid. Stir each mixture and determine whether the
solid is soluble in each solvent at room temperature. If the unknown
is not soluble in a given solvent, place the test tube in the hot
water bath. Stir or swirl the tube, observing whether the solid is
soluble in hot solvent. Allow the hot solutions to cool slowly to
room temperature. If crystals form in the cooled mixtures, compare
their quantity, size, color, and form with the original solid
material. It may be helpful to construct a table containing the
solubility data, from which you should be able to decide the solvent
that appears best suited for recrystallization.

Once you have determined which solvent will be most effective for
recrystallizing your unknown sample, dissolve your sample in a minimum
amount of hot solvent. Save a spatulaful of your impure sample for
seed crystals and to compare its melting point to your recrystallized
sample. Using an Erlenmeyer flask, add the hot solvent to the solid.
Do NOT add the solid to the hot solvent. Hot filter the sample if
necessary. Allow the solution to cool slowly to allow crystals to
form. Once the sample has cooled to room temperature, place it in an
ice bath to complete the crystallization. If necessary, you may
induce crystallization by scratching the inside of the flask with a
stirring rod (rough end), cooling your sample in an ice bath, or
adding a seed crystal. If your sample oils out, heat it up again and
add more solvent before allowing it to cool again. Collect the
crystals by vacuum filtration using a Hirsch funnel. Wash the
crystals with a small portion of cold solvent. Allow the crystals to
air-dry overnight before determining the weight and melting point of
your purified sample. For your lab report, calculate the percent
recovery of the pure crystals that you obtain. Comment on the purity
of your crystals and on the percent recovery.